1. Field of the Invention
This invention relates to thermal imaging devices, and in particular to optically-read thermal imaging devices.
2. Description of Related Art
Conventional thermal imaging devices include an infra-red sensitive detector and one or more infra-red optical components which focus the radiation from a thermal scene on to the surface of the detector. Devices of this type are described in patent application Ser. No. GB 2150387 and in our own patent application Ser. No. GB 2180361. The material of the detector is chosen to have a highly temperature-dependent optical property, such as birefringence or optical rotation. This detector is also illuminated by a visible or near-visible polarized light source the output of which is reflected off a dichroic mirror on to the detector. The visible or near-visible light experiences a modulation of polarisation, on passing through the detector, which is converted to an intensity modulation by a quarter-wave plate and an analyser. This intensity modulation conferred on the light passing through the detector thus corresponds to temperature variations induced in the detector by the infra-red radiation. In conventional devices, the modulated light is then focused by lenses through a Fourier plane filter on to a television camera chip the output of which is read into a frame store.
In order to vary the field of view of the device, an infra-red zoom lens must be used to focus the radiation from the thermal scene on to the liquid crystal. Infra-red lenses are expensive, and infra-red zoom lenses particularly so; so for any individual system, the range of lenses is usually limited.